Memory devices are very complex. Even the smallest process variations can lead to improperly functioning devices. As memory components become smaller, more process variation occurs.
Memory read operations are particularly affected by process variation. Too much process variation can result in an inaccurate read operation.
Conventionally, a sense amplifier reads voltage levels from two bit lines (a bit line and a bit line bar) connected to the memory cell being read. The sense amplifier analyzes the difference between the two bit line voltages to determine what value is stored in the memory cell. The difference is known as a sense margin (or read margin).
The sense margin needs to be large enough to guarantee a proper read operation. With smaller memory devices, it is even more important to ensure the sense margin remains sufficient as more process variations occur with the smaller memory devices. Similarly, with lower power devices it is also more difficult to maintain a proper sense margin.
Column multiplexers within a memory device can degrade the sense margin. CMOS column multiplexers may not affect the sense margin but they have other drawbacks. For example, CMOS devices require more area than pMOS devices. Moreover, CMOS multiplexers increase the bit line capacitance.
Although pMOS column multiplexers address the above noted problems with CMOS column multiplexers, pMOS column multiplexers decrease voltage in the bit lines, reducing the sense margin. Although timing can be delayed to increase the sense margin, the delay degrades overall performance of the memory by increasing access time.
Other variables can also affect the sense margin. For example, process variation and/or operating conditions can worsen the sense margin. During the early stage of implementing a new technology, large process variations can exist. Operations in extreme or corner conditions such as a low temperature, e.g., −40 deg C., and/or low Vdd voltage, can result in read margin failures of the weak bit cells.
Thus, it would be desirable to have a memory device that does not decrease sense margins, while still addressing the problems noted above.